Globally the water production associated with the oil and gas production is more than three times higher than the oil production. This gives an average water fraction of about 75% of what is produced from the wells. The water percentage continues to increase. About ten years ago was it about 70%. The water fraction increases in proportion to the oil fields being older and decreases in proportion to better methods being developed to handle the reservoir and to new fields being introduced. Simultaneously the environmental conditions are being stricter and more complicated to meet. The challenges for the operators are increasing and the need of better, diminished and more cost effective technologies arises. The costs of water treatment participates in determining how high water content it will be profitable to produce. This will also depend on the oil price.
The purpose of the present invention is to provide a purification system for produced water with the main focus at oil installations onshore and offshore world-wide. Produced water coming up from the well with the well stream is separated from the oil and gas, and then purified and discharged to the sea or re-injected into the reservoir. Produced water is a mixture of formation water, residuals of production chemicals and re-injected water (on installations where this is carried out). The contents and composition of produced water vary from field to field and from well to well within the same field. In addition also the composition will vary over time in one and the same well. Each minute Norwegian oil platforms will treat about 400 m3 water. A constantly more mature Norwegian shelf with less oil and more water has resulted in a strong increase in produced water. In 2007 about 200 millions m3 produced water were treated on the Norwegian shelf. About 90 percent of this was discharged to the sea.
The Norwegian patent application, NO 20091364, relates to a process for separation of hydrocarbons from hydrocarbon-containing produced water, wherein in a first stage the hydrocarbon-containing produced water is supplied and mixed with a gas-containing component, whereupon the gas-and-hydrocarbon-containing produced water mixture is fed to an inlet tube in a tank, whereupon said mixture via a branch means is tangentially distributed along the tank wall via at least one tube with at least one outlet nozzle and at least one guide vane, mounted under each outlet nozzle, leading water over the next outlet nozzle, whereupon hydrocarbons and gas with adherent oil droplets will rise to at least one outlet from the tank and being discharged; and cleaned water is conveyed to an outlet in the bottom of the tank.
The process further comprises a second stage where a fraction of a gas-containing water mixture from the bottom outlet is recirculated via a stream and fed to an annulus chamber and further tangentially fed via at least one nozzle and at least one guide vane countercurrently or cocurrently to the descending water mixture from the first stage. The water mixture from first stage is fed to at least one separate stage, via at least one separating plate and at least one annulus chamber, and is further tangentially distributed via at least one nozzle and at least one guide vane. A descending water mixture may be fed over a layer of a packing material.
For large flotation tanks, above ca 50 m3, the process and apparatus of NO20091364 have not performed optimally because the water stream from one distribution tube has not been properly mixed with the incoming water from the next distribution tube.
To obtain optimal mixing and consequently optimal cleaning of the produced water, the inventor has found that shrouds or annular walls arranged on the inside of the guide vane(s) 8.1, in an angle of 30° to 150° in relation to guide vanes 8 ensure a uniform flow in the outer part of the flotation tank. The water is directed along the entire guide vane 8.1 and thus, meets the incoming flow from next distribution tube. This solution has shown to be effective for better cleaning of the process water, and is not dependent on the size of the tank.
In the present invention the stages mentioned above can be carried out in the sequence which is considered as suitable for the purpose of the invention. Further the various stages can be repeated a number of times if it is considered suitable.